The present invention pertains to an improved device for measuring the surface insulation characteristics of coatings on magnetic materials.
In January, 1947 R. F. Franklin's article entitled "Measurement and Control of Interlaminar Resistance of Laminated Magnetic Cores" appeared in the ASTM Bulletin. This article describes an instrument developed to test, under simulated operating conditions, insulating films applied to sheet steel to determine if such films are adequate. In recognition of its developer such instrument became well known as the Franklin tester.
Prior to the development of the Franklin tester the recognized method of testing the insulating value of interlaminar insulation was to apply insulating material to a number of small rectangular pieces of sheet steel. The sheets were stacked in a press and the electrical resistance between the top and bottom of the stack was measured.
The Franklin tester consists of a mounting block on which are assembled two parallel longitudinal rows of five vertically mounted steel rods free to move axially against surrounding spiral springs. At the end of each of the ten rods are brass insulated contact buttons. Each contact button is forced against the surface to be tested by the spring disposed about each cylindrical rod. In operation the Franklin tester is fastened to the head of a press. The bottom face of the contact buttons are urged against the coated surface of magnetic material to be tested at a predetermined pressure, and perhaps at a predetermined temperature, approximating the expected core pressures and temperatures to which the material will be exposed in service. The appropriate instrument circuitry is closed and the ammeter reading of between zero and 1.0 is recorded. A reading of 1.0 is indicative of poor surface film quality, while readings approaching zero indicate very good surface film quality.
The standard test method for "Surface Insulation Resistivity of Single-Strip Specimens" is set forth in ASTM Standard designation A717-75. In the described procedure the Franklin tester is attached to the head of a hydraulic press. Two drills are used to make electrical contact with the base metal to be tested. Polarity connections are made between the power supply and the Franklin tester to maintain the drills and the test specimen at a positive potential. A voltmeter is employed to indicate the voltage across the Franklin tester. The voltmeter load current appears as an error current and must be subtracted from the ammeter readings. The spring loaded, ten metallic contacts of fixed surface area are applied against one of the coated surfaces of the test specimen at a predetermined pressure. The effectiveness of the surface insulation is then indicated by a measurement of the average electrical current flowing between the contact buttons and the base metal.
Over the years, uniform contact pressure and uniform button contact area have been a source of concern to those using the Franklin tester. Such concern is evidenced by the various guidelines which have developed for the use of the Franklin tester including the following:
1. The head must be parallel to the test specimen. PA1 2. All buttons should contact the test specimen simultaneously. PA1 3. Springs must exert uniform pressure against all of the buttons as the head is applied against the test specimen. PA1 4. All ten rods, or probes, must be perpendicular to the test specimen and remain perpendicular thereto as the buttons are compressed against the test specimen. PA1 5. The buttons should not exhibit any lateral movement during a test. PA1 6. The contact surface of all buttons should exert a uniform pressure throughout the entire contact area of each button as the buttons are urged against the test specimen.
It is generally recognized that faulty alignment of the probes can cause the contact buttons to wear unevenly, can result in the nonuniform application of spring urged pressure of the buttons against the test specimen, and can produce button skidding on the test specimen which could affect the electrical current reading. All of these conditions may distort the electrical resistivity measurements indicative of the quality of the insulative coating on the magnetic material being tested.
Also, the drill pressure and drill rotation may not be consistent from test to test with the Franklin tester. Such inconsistency is due primarily to the pressure differentials that may exist because the springs do not provide a uniform bias against all of the buttons. Thus, at light Franklin test pressures of about 200 to 300 psi, there is no assurance that the drill bits cut through the coating on the test sample to establish the requisite electrical contact with the base metal.
Efforts have been made to maximize the accuracy of the results obtained from the Franklin tester. For example, motor driven drill bits have been employed, various types of articulated buttons have been utilized, such as that recently disclosed in U.S. Pat. No. 4,156,841, the tolerance on the springs has been reduced, button surfaces have been machined and cleaned, and stainless steel buttons have replaced some brass buttons. Despite such efforts, an improved apparatus for more accurately and consistently measuring the surface insulation characteristics of coatings on magnetic materials is still desired. In particular, improvements in the Franklin tester are desired which would lead to improved confidence in the test results.
The present invention may be summarized as providing an improved apparatus for measuring the surface insulation characteristics of coatings on magnetic material. Such apparatus comprises an insulative button block having a plurality of protruding contact buttons stationarily mounted therein, with each contact button provided with a substantially flat contact surface and with all contact surfaces lying substantially in the same plane. The apparatus further includes a press for urging the contact surfaces against the coated surface of the magnetic material, the electric circuitry necessary to apply a predetermined voltage potential between the contact surfaces and the magnetic material, and an ammeter to measure the current flow through the coating.
Among the advantages of the present invention is the provision of an improved Franklin test apparatus which maximizes the accuracy of the results obtained by the replacement of the often troublesome spring biased probes, with non-depressable, or fixed probes.
An objective of the present invention is to provide an improved Franklin test apparatus which yields consistent, dependable and accurate results.
Another advantage of the improved apparatus of the present invention is that the improved apparatus is more readily and easily serviced than the conventional Franklin tester.
A further objective of this invention is to provide an apparatus which may follow the established and recognized ASTM procedure for testing insulation resistivity, yet will yield more dependable measurements thereof.
These and other objectives and advantages of the present invention will be more fully understood and appreciated with reference to following detailed description and the accompanying drawings.